1. Field of the Invention
This invention relates to a method of reconstructing holograms, and more particularly to a method of reconstructing information by use of holography wherein the reconstruction efficiencies are enhanced in the reconstruction process of holography.
2. Description of the Prior Art
An object beam from an object which is illuminated with spatially coherent monochromatic illumination light and a reference beam directly obtained from the same light source of the illumination light produce an interference pattern on a photographic emulsion coated recording medium. This interference pattern recorded on the photographic recording medium is known as a hologram. In reconstructing the object information, a virtual image of the object is formed by use of a reconstructing beam having the same wavefront as that of the reference beam used in the recording process or a real image of the object is formed by use of a reconstruction beam having a conjugate wavefront with respect to that of the reference beam. The reconstruction process can be explained as a process in which the reconstructing beam is diffracted by the interference pattern recorded in the hologram and the object information is reconstructed in the first-order diffraction beams.
In the reconstruction process of holography as described above, there is the great defect that the ratio of the energy carried by the first-order diffraction beam containing object information to the whole energy of the reconstructing beam which is called "first-order diffraction efficiency" is small so that a large amount of energy is wasted as the zero-order diffraction beam passes straight through the hologram. Therefore, the image information of the object reconstructed through the holographic reconstruction process generally lacks in brightness, and accordingly, the applications of the recording and reconstruction of information by use of holography are limited.
In order to overcome the defect inherent in the above described holographic method in which the interference pattern is recorded on the medium in the form of an optical density pattern, it has been known to make a phase-only hologram in which the interference pattern is recorded in the form of a phase shift pattern. Various recording media have been used for recording the phase shift pattern. The conventional photographic emulsions can also be used for recording the phase shift pattern. In this case, developed absorptive emulsions carrying an interference pattern in the form of difference in absorption between the gelatin and silver particles are bleached to convert the silver particles to a silver salt such as silver halide. Thus, the interference pattern is recorded in the emulsion in the form of difference in the refractive index between the gelatin and silver salt. The first-order diffraction efficiency of the reconstruction of this kind of hologram, however, is about 30 percent at most even in case of reconstructing a hologram which carries information corresponding to a single point. Therefore, in the reconstruction of a normal object which carries information corresponding to an infinite number of points, the above efficiency is generally several percent.
Besides photographic emulsions, dichromated sensitized gelatin, photopolymer recording medium such as photosensitive resist, and electrooptic crystals such as lithium niobate have been used for recording the phase-only hologram. However, the dichromated sensitized gelatin is disadvantageous in that its life is short, the photosensitive resist is disadvantageous in that its sensitive wavelength is limited in the range of 200 to 500 m.mu., and the lithium niobate has a great defect in that its sensitivity is extremely low. In view of these defects, there is still a need to find proper materials for recording the phase-only hologram.